Is it normal that NCP announces my prefix with my own ASN in his upstream ASN?

I have a prefix which I announce in my own AS.

I use a BGP tunnel as well as Vultr to connect my AS. Vultr's AS20473 acts as my upstream.

Both bgp.tools and bgp.he.net show my prefix as not only being announced by my own AS but also by AS20473. The same thing does not happen with the BGP tunnel providers.

In my opinion, this is wrong: Even though Vultr is my upstream, the announcement should still only come from my AS. Do I get this wrong?

This also suggests that this is not "normal": https://bgp.tools/kb/more-than-one-asn-per-prefix

I wrote to Vultr support. Not that I would expect this to be solved by them but in their response they claim what they are doing is right:

As you engaged in peering with us, we act as your upstream provider.This is the actual BGP service we provide.Once you start announcing your prefix to us, we will forward your announcement to major transit providers in order for your prefix to be seen globally in the internet.

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I do not see why this is required or correct: The prefix itself only needs to be announced by my AS. It is the routing information (i.e., that the prefix is reachable with my AS and that my AS is reachable via AS20473) and not the prefix announcement that would need to be forwarded by Vultr.

Can anyone help me understand if they are right and what's going on here?